An actuator, in which one end side of a screw shaft is rotatably supported by a base body; a female threaded member is engaged with the screw shaft; and an actuation rod body fixed to the female threaded member can advance and retreat in the axial direction of the screw shaft, is widely used in a lifting mechanism for the bottom of a bed, etc.
On the other hand, recently for example as disclosed in Patent Document 1, it is practiced to install load sensors in all the legs of a bed so that the loads on the bed bottom can be detected as the body weight and movements of the patient to be referred to for care, nursing, etc. For example, the loads measured can be used to detect and notify that the user of a bed such as an elderly person gets off from the bed bottom, for preventing the occurrence of any accident, etc.
Means for detecting a load on a bed bottom include various means such as detecting the change of pressure in an envelope caused by a load.    Patent Document 1; JP11-342160A